In recent years, moving picture compression technologies have made significant progresses, and H.261 and H.263 of CCITT (International Telegraph and Telephone Consultative Committee) recommendation, and MPEG (Moving Picture Experts Group) of ISO (International Standards Organization) recommendation have been well known as representative international standards.
MPEG has plural standards (phases) as follows: MPEG-1 for compressing video data to 1.5 Mbps, i.e., data of a television signal to about 1/100; MPEG-2 for compressing video data so that its transmission rate becomes about 2˜15 Mbps, which is the standard of current digital television broadcasting that realizes the current television quality; and MPEG-4 which enables coding and handling of video data in object units to realize new functions required for the multimedia age.
A videophone is intended and utilized as one of representative applications of H.261, H.263, and MPEG-4 among the above-mentioned standards. In the videophone, an image composition technique called “picture in picture (PinP)”, in which a picture (sub-screen) of a local station (a calling user) is displayed in a picture (main screen) of an opposed station (a called user), is well used.
Further, in the digital television broadcasting as a representative application of MPEG-2, there are many cases where a program table called “EPG (Electric Program Guide)” is displayed in a received image, or plural received programs are displayed in a single received image.
Furthermore, also in the current analog television broadcasting, subtitles such as current time or news flash are generally superimposed on a received image.
As described above, techniques for compositing plural pictures and displaying a composite picture have been frequently employed, and further, there are many cases where a frame for isolating each image to clearly display the image is added to the image.
As a method for adding such frame, there has commonly been employed a technique called “OSD (On Screen Display)” in which a frame is added to an image in a stage subsequent to an image composition stage, or a technique in which a means for writing a graphic pattern for forming a frame is additionally provided, and a frame is added to a display image by compositing the graphic image and the display image.
It is one of features of MPEG-4 that an image signal is coded in object units, and MPEG-4 can handle arbitrary-shaped objects as well as rectangle objects.
An image signal corresponding to each object is composed of a shape signal indicating the shape of the object, and a texture signal indicating the texture of the object. When decoding the image signal, decoding of the shape signal and decoding of the texture signal are respectively carried out by different decoding methods.
Then, composition of a foreground image and a background image is carried out using the respectively decoded shape signal and texture signal, thereby generating a composite display image.
In the conventional technique, however, when it is tried to add a frame or an outline to a specific object in a composite image including arbitrary-shaped objects, since the shape of the object or the object itself cannot be recognized, an accurate frame or outline cannot be added to the object.
Furthermore, in the conventional technique, since it is premised that images to be composited are rectangle, the shape of a frame to be added is also restricted to rectangle.
As described above, according to the conventional image composition method and apparatus, in the image signal processing wherein coding is carried out in object units like the MPEG-4 standard, when an image having an arbitrary shape is composited with another image, it is difficult to add an outline or an arbitrary-shaped frame to the arbitrary-shaped image.